This invention relates to a method of evaluating the quality of carbon composite parts, and especially to improvements in testing the oxidation resistance of composite materials at high temperatures.
Composite materials made of graphite fibers embedded in a carbon matrix have excellent high temperature strength characteristics and are potentially useful in jet engines. They may be fabricated into light weight structures for the sections of engines that must withstand operating temperatures of 2000.degree. C. and more. This material retains its strength at much higher temperatures than conventional composites as long as it is protected from the oxygen in air; it burns away if exposed to oxygen at the desired operating temperature. Practical carbon composite parts must be covered with protective coatings which inhibit this destructive oxidation. The oxidation resistance of coated composite parts may be further enhanced by incorporating antioxidants, typically boron components, in the bulk material.
Both the initial selection of a protection system and subsequent verification of its application to individual parts hinges on the development of an adequate method of evaluating the oxidation of coated carbon composite samples. Traditionally the effectiveness of a given anti-oxidation system has been evaluated by the destructive testing of small coupons in a gravimetric oven. The sample is heated for many hours in an oxidizing atmosphere and its progressive weight loss is recorded. As carbon combines with oxygen to form carbon dioxide, the material loses carbon and its strength decreases. Those samples that lose weight are judged to be unacceptable. Such a test takes a long time, sometimes days, and may severely damage the part.